WASHINGTON, May 31 -- The number of Islamic State (IS) fighters is swelling as more IS recruits leave their home countries to join the group, which means a tougher job for the United States in the fight against the IS.

The IS has overtaken vast swaths of territory in Syria and northern Iraq in recent months, and a UN report released earlier this month found that the ranks of groups like al-Qaeda and IS may have increased by more than 70 percent over the last nine months, with fighters coming from around 100 different countries. There are currently around 25,000 such foreign terrorist fighters, according to the report.

That could spell trouble for the U.S.-led coalition that aims to destroy the IS, not only because there will be more fighters on Middle East battlefields, but also because those fighters could one day return to their home countries with specialized training and wreak havoc at IS' behest.

"I think this massive increase of foreign fighters into Iraq and Syria to fight with the IS is troubling on two fronts. First, the more fighters the IS has, the more difficult it will be to defeat on the battlefield. More fighters mean the ability to cover more territory, which increases the capacity for taxation and extortion in these areas, financing the organization and enabling its activities," RAND Corporation's associate political scientist Colin P. Clarke told Xinhua.

"Secondly, the more foreign fighters going to fight for the IS, the greater overall number of fighters that will likely return to their countries of origin, at some point. Many of those will be detected and detained, but it is also possible that some may return unbeknownst to the law enforcement and the intelligence services in the West," he said.

Indeed, earlier this month saw the first-ever IS-related attack on U.S. soil as two gunmen opened fire on a cartoon event in the U.S. state of Texas.

Information about the exact involvement of the IS remains under investigation, but the Islamist group did take credit for the attack on a contest to draw cartoon images of the Prophet Muhammad. Making any image of the Prophet Muhammad is considered blasphemy by many Muslims, and the IS believes it is a crime punishable by death.

Clarke said the more territory the IS holds, the easier it will be for the group to use areas as its sanctuary and safe haven to train its fighters. Still, U.S. airpower means that when the IS trains out in the open, its fighters are more vulnerable.

Indeed, the U.S.-led air campaign is an effort to prevent Washington's worst nightmare -- another massive terror attack on U.S. soil on the scale of the Sept. 11, 2001 strike that killed around 3,000 people in New York and Washington.

Wayne White, former deputy director of the U.S. State Department's Middle East Intelligence Office, told Xinhua that despite more terrorists being trained by the IS, it seems that the number of those returning home has not been particularly high, so an ironic upside of accelerated departures from home countries is that it cuts down the numbers of IS-inspired potential terrorists back home, although a downside of all this is the reinforcement of IS military capability in its core holdings.

The UN report also found that most countries it has looked at have done little to prevent jihadists from traveling to Syria and linking up with the IS fighters there. Most fighters are males aged between 15 and 35 and are motivated by ideology, the report found, but added that there are also women joining the militants.

Meanwhile, critics fret the U.S. strategy in Iraq may not be working, after IS radicals in recent weeks took the Iraqi city of Ramadi, casting doubt on whether the U.S.-led collation can destroy the Islamist terror group.

For its part, the White House described the fall of Ramadi as a setback, but critics said that word underplays the significance of IS' tactical victory. Moreover, pictures of the Iraqi army evacuating the area have also a huge symbolic and propaganda value to the IS.

While U.S. President Barack Obama's strategy entails building capacity in Iraq's army, bombing IS targets and reconciling relations between Shiites and Sunnis, the latter has so far seen little success. While Iraqi forces are now mounting a counter-offensive to take back the city, experts said this may turn out to be a long, drawn out war.